U.S. defense officials say two B-52 bombers flew near contested islands in the South China Sea earlier this week, and received a verbal warning from Chinese ground controllers in the latest instance of Washington challenging Beijing's expansive territorial claims there.

The bombers, which originated from and returned to a U.S. air base on Guam, conducted a "routine mission in international airspace in the vicinity of the Spratly Islands" on November 8 and 9, according to Commander Bill Urban, a Pentagon spokesman on Thursday.

The planes "received two verbal warnings from a Chinese ground controller despite never venturing within 15 nautical miles [28 kilometers] of any feature," Urban said. "Both aircraft continued their mission without incident, and at all times operated fully in accordance with international law."

Pentagon Press Secretary Peter Cook also confirmed the flight, which he said was not out of the ordinary. "I know we conduct B-52 flights in international airspace in that part of the world all the time," he said during a press conference Thursday.

Chinese Foreign Ministry spokesman Hong Lei on Friday said Beijing opposes "the action of undermining China's sovereignty and security under the pretext of freedom of navigation and overflight."

The U.S. military has been ramping up what it calls routine "freedom of navigation" operations in the South China Sea, where Beijing has competing territorial claims with Brunei, Malaysia, the Philippines, Taiwan and Vietnam.

FILE - USS Lassen (DDG 82) operates in international waters near the Chinese People's Liberation Army (Navy) Jianghu V-class frigate Dongguan (560) while on patrol in U.S. 7th Fleet at South China Sea, Sep 29, 2015.

In the boldest U.S. move yet, the USS Lassen last month sailed within 11 kilometers of the Subi Reef in the Spratly Island archipelago. Beijing launched a massive building project last year to transform the submerged reefs into islands that can support runways and other facilities.

The artificial island building project has outraged China's neighbors who are concerned Beijing will use the facilities in part to enforce its disputed claims to the area. The U.S. has called on China to stop the island-building.

The maritime tensions are expected to be a major focus next week when U.S. President Barack Obama travels to the region to meet with regional leaders at a pair of Asia-Pacific summits.

Although Washington says it does not take an official position on the territorial disputes, U.S. leaders routinely slam Chinese territorial claims in the South China Sea and have developed closer military ties with many of China's rival claimants.